Bifunctional metal-organic frameworks afforded by postsynthetic modification for efficient cycloaddition of CO2 and epoxides

Bifunctional ionic metal-organic frameworks (MOFs) containing Lewis acid sites (unsaturated metal sites) and halide ions (Cl-, Br-, and I-) have attracted increasing attention due to their extra high activity for the cycloaddition of CO2 with epoxides. Herein, a novel microporous MOF (1-Eu), namely, [Eu-3(L)(2)(HCOO)(H2O)(5)]center dot 14H(2)O (H4L = 2,6-di(2,4-dicarboxyphenyl)-4-(pyridine-4-yl)pyridine), has been synthesized by using a new bipyridyl-based tetracarboxylate ligands (H4L). Structural analyses show that 1-Eu is a 3D framework in which 1D chains with alternating triple and single carboxylate bridges are interlinked by the L ligands and contains microporous channels with uncoordinated pyridyl N atoms, which are easy to be modified by N-methylation. Therefore, three bifunctional N-methylation 1-Eu MOFs, 1-Eu-MeX (X = Cl-, Br-, and I-), were successfully prepared from the 1-Eu MOF by a postsynthetic modification (PSM) method. 1-Eu-MeX can efficiently catalyze the cycloaddition reaction without any cocatalyst and solvent. Among them, the 1-Eu-MeI catalyst displays the highest catalytic performance. Our work thus represents a rare demonstration of ionic MOFs as heterogeneous catalysts for efficient CO2 fixation with epoxides. More significantly, 1-Eu-MeX are the first reported Eu-based ionic MOFs with the bipyridyl-based tetracarboxylate ligand.